Pressure sensor devices have been used to study various physiological conditions in biomedical applications. One known application is to monitor intraocular pressure, for example, in connection with treatment of glaucoma. Glaucoma is a well known ocular disease that affects millions of people. Persons afflicted with this disease require treatment for life. The disease causes visual field loss and if left untreated, may result in permanent loss of vision, and is a primary cause of blindness in the United States and elsewhere.
The exact cause of glaucoma is not known, but it is characterized by pathological changes in the optic disc and nerve fiber of the retina. Studies suggest that development of the disease may be attributable to various factors including elevated intraocular pressure. Normal intraocular pressure typically ranges from about 10 to about 21 mm Hg, e.g., about 15 mm Hg. Intraocular pressures of eyes of patients having glaucoma often exceed 21 mm, although glaucoma may exist when intraocular pressures are at lower levels. Elevated intraocular pressures are believed to be responsible for slowly damaging the optic nerve which, in turn, can cause blind spots in the field of vision. Total blindness may occur if the entire optic nerve is damaged.
One known manner of measuring intraocular pressure is to use an external pressure measurement device that acquires intraocular pressure readings from outside of the eye. One known pressure measurement device is known as a tonometer, which measures an external deformation of an eye and relates that measurement to intraocular pressure. Such external measurement devices, however, may not have the desired level of accuracy since they operate in an external environment rather than within the eye itself. Further, such devices do not provide for continuous monitoring of intraocular pressure since a tonometer must be utilized each time intraocular pressure is to be determined and, therefore, provides discontinuous intraocular pressure monitoring.
It is also known to implant a sensor into an eye for purposes of measuring an electrical parameter related to intraocular pressure, and to use telemetry to obtain an electrical parameter measurement and relate the electrical parameter measurement to intraocular pressure. In one known system, an external instrument generates a signal to remotely energize an in vivo intraocular pressure sensor. The response generated by the in vivo sensor is measured and correlated to intraocular pressure.
For example, referring to FIG. 1, a known intraocular telemetry system 10 includes an external system 20 and an internal or implanted intraocular sensing circuit 30. The external system 20 includes an excitation circuit 21 and a measurement device 22. The sensing circuit 30 typically includes a resistor (Rsensor) 33 and an inductor (Lsensor) 34 and a capacitor (Csensor) 35. The capacitor 35 may be configured to vary with the intraocular pressure applied to the capacitor 35.
The excitation circuit 21 typically includes an inductor (L) 24. During use, the excitation circuit 20 generates energy, which is delivered to the sensing circuit 30 by inductive coupling between the inductors 24, 34, thereby energizing the sensing circuit 30. The resulting response (e.g., resonant frequency or impedance) of the sensing circuit 30 is measured by the measurement device 22 and correlated to intraocular pressure.
The implanted sensing circuit 30 is essentially an RLC resonance circuit. The resonant frequency and the Quality (Q) factor of the circuit 30 are determined by resistance, capacitance and inductance parameters as provided by Resonant Frequency (f)=1/(2π√(LC)); and Q Factor=1/R (√(L/C)). A change of capacitance causes a shift in resonant frequency of the implanted sensor circuit 30, which can be wirelessly measured by the external measurement device 22. Examples of such intraocular implants and telemetry systems are described in U.S. Pat. No. 6,579,235 to Abita et al., “Passive Silicon Transensor Intended for Biomedical, Remote Pressure Monitoring,” by Backlund et al., “A system for wireless intra-ocular pressure measurements using a silicon micromachined sensor,” by Rosengren et al., and “A system for passive implantable pressure sensors”; by Rosengren et al.
One known capacitor for use in intraocular pressure sensors is manufactured using MEMS technologies and includes a membrane, a flat bottom portion and a chamber. The capacitor is part of a pressure sensor that is implantable to monitor pressures through a remote telemetry connection. Another known capacitor device used in pressure sensors is referred to as a comb-drive capacitor unit. One known capacitor unit is described in “Design and Simulation of a MEMS-Based Comb-Drive Pressure Sensor for Pediatric Post-Operative Monitoring Applications,” by Duck-Bong Seo et al. Seo et al. describe an implantable MEMS-based pressure sensor to monitor pressures through a remote telemetry connection in the context of monitoring pressures of the right side of the heart following surgery. Seo et al. show a flat membrane and a comb drive and explain that a change of overlapping area changes the capacitance of the device, and that no bending or other deformation of the membrane was found for the comb-drive sensor.
While known sensor devices and telemetry systems may provide some improvements over known external pressure measurement devices, they can be improved. For example, certain known sensor devices present performance, biocompatibility, packaging and/or size challenges. Certain known devices also lack sensitivities and detection ranges suitable for various biomedical applications. Further, certain known devices utilize wafer bonding techniques, which typically require additional fabrication time and result in larger or thicker devices. Additionally, bonding often results in reduced yield rate, e.g. due precise component alignment requirements. Thus, devices that are fabricated using wafer bonding are not desirable. Certain known devices also may not be adaptable to commercial fabrication on a large scale. Additionally, the inductor element of the implanted sensor circuit can be improved to provide a more effective sensor circuit and more accurate intraocular pressure determinations. Known devices may also require larger incisions or blades for implantation of sensor devices due to their large size. Such incisions are not desirable. Further, certain known implants require sutures to remain implanted in the eye, which are also not desirable.
Therefore, it would be desirable to have implantable sensor devices that can be fabricated using known micromachining and MEMS technologies. It would also be desirable to have implantable sensor devices that are sufficiently small or miniature in size so that they may be delivered through a needle rather than through a large incision using a blade. It would also be desirable to have sensor devices that may be implanted without the need for sutures and in various locations of an eye. Further, it would also be desirable to have biocompatible and implantable microfabricated sensor devices with improved capacitor and inductor components for enhanced sensitivity, dynamic range and accuracy. It would also be desirable to continuously and passively monitor intraocular pressure by telemetry using such sensor devices. Such capabilities would enhance biomedical applications and pressure-dependent physical conditions and diseases including monitoring of intraocular pressure.